Understanding xmlchemy

xmlchemy is an object-XML mapping layer somewhat reminiscent of SQLAlchemy, hence the name. Mapping XML elements to objects is not nearly as challenging as mapping to a relational database, so this layer is substantially more modest. Nevertheless, it provides a powerful and very useful abstraction layer around lxml, particularly well-suited to providing access to the broad schema of XML elements involved in the Open XML standard.

Additional topics to add ...

  • understanding complex types in Open XML
  • understanding attribute definitions Open XML
  • understanding simple types in Open XML

Adding support for a new element type

  • add a new custom element mapping to pptx.oxml.__init__
  • add a new custom element class in the appropriate pptx.oxml subpackage module
  • Add element definition members to the class
  • Add attribute definition members to the class
  • Add simple type definitions to pptx.oxml.simpletype

Example

from pptx.oxml.xmlchemy import BaseOxmlElement


class CT_Foobar(BaseOxmlElement):
    """
    Custom element class corresponding to ``CT_Foobar`` complex type
    definition in pml.xsd or other Open XML schema.
    """
    hlink = ZeroOrOne('a:hlink', successors=('a:rtl', 'a:extLst'))
    eg_fillProperties = ZeroOrOneChoice(
        (Choice('a:noFill'), Choice('a:solidFill'), Choice('a:gradFill'),
         Choice('a:blipFill'), Choice('a:pattFill')),
        successors=(
            'a:effectLst', 'a:effectDag', 'a:highlight', 'a:uLnTx',
            'a:uLn', 'a:uFillTx' 'a:extLst'
        )
    )

    sz = OptionalAttribute(
        'sz', ST_SimpleType, default=ST_SimpleType.OPTION
    )
    anchor = OptionalAttribute('i', XsdBoolean)
    rId = RequiredAttribute('r:id', XsdString)

Protocol

>>> assert isinstance(foobar, CT_Foobar)
>>> foobar.hlink
None
>>> hlink = foobar._add_hlink()
>>> hlink
<pptx.oxml.xyz.CT_Hyperlink object at 0x10ab4b2d0>
>>> assert foobar.hlink is hlink

>>> foobar.eg_fillProperties
None
>>> foobar.solidFill
None
>>> solidFill = foobar.get_or_change_to_solidFill()
>>> solidFill
<pptx.oxml.xyz.CT_SolidFill object at 0x10ab4b2d0>
>>> assert foobar.eg_fillProperties is solidFill
>>> assert foobar.solidFill is solidFill
>>> foobar.remove_eg_fillProperties()
>>> foobar.eg_fillProperties
None
>>> foobar.solidFill
None

OneAndOnlyOne element declaration

The OneAndOnlyOne callable generates the API for a required child element:

childElement = OneAndOnlyOne('ns:localTagName')

Unlike the other element declarations, the call does not include a successors argument. Since no API for inserting a new element is generated, a successors list is not required.

Generated API

childElement property (read-only)
Holds a reference to the child element object. Raises InvalidXmlError on access if the required child element is not present.

Protocol

>>> foobar.childElement
<pptx.oxml.xyz.CT_ChildElement object at 0x10ab4b2d0>

RequiredAttribute attribute declaration

reqAttr = RequiredAttribute('reqAtr', ST_SimpleType)

Generated API

childElement property (read/write)
Referencing the property returns the type-converted value of the attribute as determined by the from_xml() method of the simple type class appearing in the declaration (e.g. ST_SimpleType above). Assignments to the property are validated by the validate() method of the simple type class, potentially raising TypeError or ValueError. Values are assigned in their natural Python type and are encoded to the appropriate string value by the to_xml() method of the simple type class.

ZeroOrOne element declaration

childElement = ZeroOrOne(
    'ns:localTagName', successors=('ns:abc', 'ns:def')
)

Generated API

childElement property (read-only)
Holds a reference to the child element object, or None if the element is not present.
get_or_add_childElement() method
Returns the child element object, newly added if not present.
_add_childElement() empty element adder method
Returns a newly added empty child element having the declared tag name. Adding is unconditional and assumes the element is not already present. This method is called by the get_or_add_childElement() method as needed and may be called by a hand-coded add_childElement() method as needed. May be overridden to produce customized behavior.
_new_childElement() empty element creator method
Returns a new “loose” child element of the declared tag name. Called by _add_childElement() to obtain a new child element, it may be overridden to customize the element creation process.
_insert_childElement(childElement) element inserter method
Returns the passed childElement after inserting it before any successor elements, as listed in the successors argument of the declaration. Called by _add_childElement() to insert the new element it creates using _new_childElement().
_remove_childElement() element remover method
Removes all instances of the child element. Does not raise an error if no matching child elements are present.